In chapter 9 of Freak the Mighty, Freak has Max take him to a medical research building. Here, Freak discloses that the doctors are doing experiments on him to make him bionic. Max does not understand what this means, so Freak explains that he will become a human robot. Freak makes Max swear not to tell anyone, because these experiments are top secret. When Max wonders if these experiments might be dangerous, Freak answers, "Life is dangerous."
This interaction between Freak and Max is recalled after Freak's death. It is explained to Max that Freak's "secret" was really a lie he told to give Max and himself hope that his condition would be cured. In Freak's fantasy, he becomes bionic, eliminating his physical weaknesses so that he can live a full life. Though this does not come to pass, we see the similarities to Freak and Max's joint identity of Freak the Mighty. Freak "drives" Freak the Mighty, and Max carries out the actions. Their joint identity allows Freak to use a different body to achieve things, similar to his fantasy about becoming bionic. Similarly, Freak the Mighty gives Max a brain, something he repeatedly says he lacks prior.
Though Freak's secret turns out to be a lie, this interaction characterizes the friendship between Freak and Max and reveals Freak's secret desire.